An expander has been known as a fluid machine to be used for the purpose of recovering internal energy of the pressure drop of a refrigerant in a refrigeration cycle from a high pressure to a low pressure along with the expansion of the refrigerant. A mechanical power recovery type refrigeration cycle apparatus using a conventional expander will be described below.
FIG. 7A shows a conventional mechanical power recovery type refrigeration cycle apparatus. This refrigeration cycle apparatus includes a compressor 1, a gas cooler 2, an expander 3, an evaporator 4, a rotation motor 5, and a shaft 6 for directly coupling the compressor 1, the expander 3 and the rotation motor 5. Carbon dioxide is used as a refrigerant which is a working fluid. The refrigerant is compressed in the compressor 1 to a high temperature and high pressure state, and thereafter is cooled in the gas cooler 2. The refrigerant further is subjected to pressure drop to a low temperature and low pressure state in the expander 3, and thereafter is heated in the evaporator 4. The expander 3 recovers the internal energy of the pressure drop of the refrigerant from a high pressure to a low pressure along with the expansion thereof, converts the recovered energy into the rotation energy of the shaft 6, and uses it as a part of energy for driving the compressor 1. Thus, the power consumption of the rotation motor 5 is reduced.
In the above-mentioned mechanical power recovery type refrigeration cycle apparatus, the compressor 1 and the expander 3 are coupled directly by the shaft 6. Since the compressor 1 and the expander 3 rotate at the same rotation speed, the refrigeration cycle apparatus is subjected to a so-called constraint of constant density ratio, in which the ratio between the specific volume of the suction refrigerant in the compressor 1 and the specific volume of the suction refrigerant in the expander 3 or the ratio between the density of the suction refrigerant in the compressor 1 and the density of the suction refrigerant in the expander 3 is fixed to the ratio between their suction capacities. This constraint makes it impossible to perform optimal pressure and temperature control, which causes a problem of reduction in COP (Coefficient of Performance).
JP 2004-150748 A discloses a mechanical power recovery type refrigeration cycle apparatus in which injection is performed in order to avoid the above-mentioned constraint of constant density ratio. The configuration of the refrigeration cycle apparatus is shown in FIG. 7B. According to this configuration, at the outlet side of the gas cooler 2, the passage of a refrigerant branches into two: a suction passage 9A; and an injection passage 9B. A portion of the refrigerant flows into the suction passage 9A, passes through a pre-expansion valve 7, and is drawn into the expander 3, while the remaining portion of the refrigerant flows into the injection passage 9B, passes through an adjusting valve 8, and then is introduced into a working chamber (not shown) in the expansion process in the expander 3. For the purpose of avoiding the constraint of constant density ratio, this mechanical power recovery type refrigeration cycle apparatus controls the opening degree of the pre-expansion valve 7 and the adjusting valve 8 so as to change the specific volume of the refrigerant to be drawn into the expander 3.
JP 2006-46222 A discloses a single-stage rotary expander and a two-stage rotary expander to be used in a mechanical power recovery type refrigeration cycle apparatus in which injection is performed. The configurations of these rotary expanders are shown in FIGS. 8A and 8B. According to the single-stage rotary expander as shown in FIG. 8A, an opening degree adjustable throttle valve 13 is provided in an injection passage 12 branching off a suction passage 11, and an introduction outlet 15 of the injection passage 12 leading to a working chamber 16 is provided on the inner circumferential surface 14 of a cylinder. On the other hand, according to the two-stage rotary expander as shown in FIG. 8B, an opening degree adjustable throttle valve 23 is provided in an injection passage 22 branching off a suction passage 21, and an introduction outlet 27 of the injection passage 22 leading to a working chamber 28 is provided at a position that is tangent to the inner circumferential surface 24a of the first cylinder 24, on a closing member (not shown) for closing the working chamber 28 at the side of the first cylinder 24.
However, the above-mentioned conventional rotary expander, in which the introduction outlet of the injection passage is provided on the inner circumferential surface of the cylinder or at the position that is tangent to the inner circumferential surface thereof, has the following problems. As shown in FIGS. 8A and 8B, when a piston is in the vicinity of the top dead center, the injection passages 12, 22 respectively are communicated with discharge passages 17, 30 through the working chamber 16, and the working chambers 28, 29 and the communication passage 26, and the working fluid leaks from the injection passages 12, 22 into the low-pressure discharge passages 17, 30. The conventional expander cannot recover the expansion energy of the working fluid that has leaked, which causes a problem of the efficiency of the expander being degraded.